A sustain driver to drive a sustain electrode in a plasma display panel for example is known as a conventional driving circuit to drive a capacitive load.
FIG. 13 is a circuit diagram showing the configuration of a conventional sustain driver. As shown in FIG. 13, the sustain driver 400 includes a recovering capacitor C11, a recovering coil L11, switches SW11, SW12, SW21, and SW22, and diodes D11 and D12.
The switch SW11 is connected between a power supply terminal V4 and a node N11, while the switch SW12 is connected between the node N11 and the ground terminal. The power supply terminal V4 is provided with voltage Vsus. The node N11 is connected to 480 sustain electrodes for example, and in FIG. 13 a panel capacitance Cp corresponding to the total capacitance between a plurality of sustain electrodes and the ground terminal is shown.
The recovering capacitor C11 is connected between a node N13 and the ground terminal. The switch SW21 and the diode D11 are connected in series between the nodes N13 and N12, and the diode D12 and the switch SW22 are connected in series between the nodes N12 and N13. The recovering coil L11 is connected between the nodes N12 and N11.
FIG. 14 is a timing chart for use in illustration of the operation of the sustain driver 400 in FIG. 13 during a sustain period. FIG. 14 shows the voltage at the node N11 and the operation of the switches SW21, SW11, SW22 and SW12 in FIG. 13.
At first, during the period Ta, the switch SW21 turns on, and the switch SW12 turns off. At the time, the switches SW11 and SW22 are both off. Thus, LC resonance by the recovering coil L11 and the panel capacitance Cp causes the voltage at the node N11 to gradually rise. During the period Tb, the switch SW21 turns off, and the switch SW11 turns on. Thus, the voltage at the node N11 abruptly increases, and the voltage at the node N11 is fixed at the level of Vsus during the period Tc.
During the period Td, the switch SW11 turns off, and the switch SW22 turns on. Thus, the LC resonance by the recovering coil L11 and the panel capacitance Cp causes the voltage at the node N11 to gradually decrease. Then, during the period Te, the switch SW22 turns off, and the switch SW12 turns on. Thus, the voltage at the node N11 abruptly drops, and is fixed at the ground potential level. The above operation is repeated during the sustain period, so that a periodic sustain pulse Psu is applied to the plurality of sustain electrodes.
As described above, the rising and falling parts of the sustain pulse Psu consist of the LC resonance part during the periods Ta and Td by the operation of the switch SW21 or SW22 and edge parts e1 and e2 during the periods Tb and Te by the turn-on operation of the switch SW11 or SW12.
These switches SW11, SW12, SW21 and SW22 are each composed of an FET (field effect transistor) serving as a switching element, and each FET has a drain-source capacitance as a parasitic capacitance, and a line connected to each FET has an inductance component. Therefore, when the switch SW11 or the like changes from an off state to an on state, LC resonance is generated by the drain-source capacitance and the inductance component of the lines, and the LC resonance causes unwanted electromagnetic wave radiation.
The diodes D11 and D12 each have an anode-cathode capacitance as a parasitic capacitance, and a line connected to each diode has an inductance component. Therefore, when the switch SW11 or the like changes from an off state to an on state, LC resonance is generated by the anode-cathode capacitance and the inductance component of the lines, and the LC resonance causes unwanted electromagnetic wave radiation.
Furthermore, the drain-source capacitance of each FET, the anode-cathode capacitance of each diode and the inductance component of each line are small, so that the LC resonance frequency is high, and the frequency of the resultant electromagnetic wave is also high. Meanwhile, according to the standard for unwanted radiation defined by the Electrical Appliance and Material Control Law (Federal Communications Commission (FCC) in the United States), a limit value is set for an electromagnetic wave having a frequency of 30 MHz or higher. As a result, the radiation of such a high frequency electromagnetic wave could have an electromagnetically adverse effect on other electronic devices, and therefore the radiation of such an unwanted, high frequency elecdomagnetic wave should be suppressed.